Rabbit mouse hybridomas (RMH) have been used in a number of studies concerning the structure and synthesis of rabbit immunoglobulin chains. This study has involved determination of conditions for stabilization of RMH and determination of growth conditions for maximal yields of mRNA. The rabbit L chain product of RMH 12F2 has been characterized by protein sequence analysis. This product could be precipitated from cell-free translates using mRNA from the 12F2 cell line. Following enrichment the mRNA was used to prepare cDNA which is full length copy including the variable region and leader sequence of this chain. An RMH producing a rabbit heavy chain is now under study. Additional studies are directed towards determining whether rabbit DNA integrates into the mouse myeloma cells randomly or at predetermined sites in the mouse genome.